Motorcycle aerodynamics, a slippery and windy, slope?

[Frank Lee]

Vetter has had quite a few different tails on his yellow scooter but IIRC he hasn't talked about the pros and cons of each iteration all that much.

[sendler]

Originally Posted by Frank Lee

Vetter has had quite a few different tails on his yellow scooter but IIRC he hasn't talked about the pros and cons of each iteration all that much.

So after all my fretting and thinking my contentions would be soundly trounced, it looks like Vetter has recently chopped off his tail as I would have wanted on my streamliner if I had one. The pic with the long tail is what he had been using and the latest pic shows his new nose photoshopped onto his bike and the tail is chopped off.

[jkv357]

Interesting. That's the first time I've seen the cut-off tail on Craig's Helix, and I follow his work pretty closely.

I remember seeing illustrations here that show there isn't a huge aero penalty for not running the tail shape out completely until it closes up.


Jay

[Ken Fry]

Originally Posted by sendler

So after all my fretting and thinking my contentions would be soundly trounced, it looks like Vetter has recently chopped off his tail as I would have wanted on my streamliner if I had one.

In this side view of an early version of the Zing, if you imagine a single front motorscooter wheel, you'd have an earlier yet version, which used the same fuselage. My reasoning for truncating the rear was to reduce side area and to keep the center of lateral resistance somewhere near the middle of the bike. This allows the self-correcting for cross winds to continue to work. A long tail tends to pivot the bike about the rear wheel, so that the self correcting we've been discussing goes away or is (worse) reversed. (I didn't get enough hours on the two wheel version to verify if self-correction in crosswinds worked, nor enough to evaluate handling in really strong crosswinds.)

Now that the car is three-wheeled, I've extended the tail to reduce the wake -- the Kamm effect (little or no adverse effect on drag by removing the back end) is not valid if flow is fully attached at the point where you cut the back off. In a three wheeler (or four wheeler) where you steer right to go right (imagine!) then having area aft tends to turn the car into the crosswind, which is stabilizing.

[Ken Fry]

Originally Posted by sendler

The tail fin can only push the rear of the bike with the wind but it is forcing the rear tire to go sideways in order for anything to happen.

Because the rear wheel cannot move sideways easily, instead the steering head will be pushed into the wind, cancelling (to some extent) the bike's self correction.

I quickly scanned the web but could not find Wagner's justification for the rudder option. But if I recall from having read it years ago, I think it was intended mainly for roll control assist. The fact that it is rarely seen, and is not seen as an option for the Monotracer, suggests that it has little value. I haven't seen a close up, but for it to be of any net gain, it would have to be able to weather cock, so that at very low speeds with a near direct crosswind, it does not simply tend to roll the bike over. At higher speeds, the entire thing would need to weather cock so that it is not stalled when the apparent wind angle is greater than about 12 degrees... if you are using it for roll control, then you'd want to be well away from the stall regime all the time.

In aircraft, stabilators (combined horizontal stabilizer and elevator) are not uncommon. Combined vertical stabilizers and rudders are very uncommon -- but for a motorcycle, you'd need something of this nature, for the many conditions when you don't want something high up trying to push you over. Then, unless the intention is to dramatically increase driver workload, the thing would need to be under computer control to interpret driver intention.

Given that $80,000 for a motorcycle is decidedly rich guy toy territory, if the rudder had any beneficial effect it would have been purchased by many customers. I'd expect it to have no useful effect, (as well as the obvious negative effect of trying to roll the bike in the wrong direction in a cross wind) and market response seems to support that.

[sendler]

I was going to say, I bet the rudder didn't stay around long.

[drmiller100]

another way to look at it. The smarter Bonneville racers understand center of pressure and center of gravity.
The faster you go, the more the center of pressure moves forward.
You ALWAYS want center of gravity in front of center of pressure, or when you get going fast enough, the dynamics of it all will fix themselves until the center of Pressure IS behind the center of gravity (Spin out).

I would think the fin might save you in a really bad spin, but it would be better to move the CG forward and the center of pressure back.

The heaviest thing on a bike is the rider. The second heaviest is the rider. The third heaviest is the engine.

Cafe racers move the riders CG forward.

[sendler]

A big tail fin helps a top speed vehicle go straight where the wind speed from the front is always way bigger than the speed of any side wind blast. On a commuter vehicle that is intended to average 70mph, a 30 mph side wind is a considerable amount of the total wind force acting on the motorcycle. For a road motorcycle, I think any added tail fin will be more of a problem than any possible improvement most of the time.
.

Originally Posted by drmiller100

another way to look at it. The smarter Bonneville racers understand center of pressure and center of gravity.
The faster you go, the more the center of pressure moves forward.
You ALWAYS want center of gravity in front of center of pressure, or when you get going fast enough, the dynamics of it all will fix themselves until the center of Pressure IS behind the center of gravity (Spin out).

I would think the fin might save you in a really bad spin, but it would be better to move the CG forward and the center of pressure back.

The heaviest thing on a bike is the rider. The second heaviest is the rider. The third heaviest is the engine.

Cafe racers move the riders CG forward.

[sendler]

Thanks Ken for another great link on steering head angle and trail. Posted in the Oregon Commuter thread. Too bad you get spread out over so many nearly identical threads.
.
Rake and Trail

[sendler]

Interesting fairing on this motorcycle for the new Moto3 250cc 4 stroke class.